Magnesium oxide is a transparent and chemically stable material having good electric insulation property and it does not undergo a phase transition even at a high temperature up to its melting temperature of 2852.degree. C. Magnesium oxide has been used as a substrate for preparing thereon films of a number of inorganic compounds, i.e., various oxides such as cuprate-based high-T.sub.c superconductors, lithium niobate, barium titanate, and nitrides such as gallium nitride, niobium nitride, and the like.
Although large single crystals having uniform properties have been successfully grown for quartz, silicon, gallium arsenide(GaAs), sapphire and the like, a process for preparing large single crystals of high-temperature superconductors has not yet been developed. Also, attempts to prepare a high-temperature superconducting film directly on the surface of quartz, silicon, gallium arsenide or sapphire crystal have not been-successful. However, it is known that a high-temperature superconductor film having excellent property can be prepared via coating a magnesium oxide film on the surface of a single crystal substrate. For example, a copper oxide high-temperature superconductor may be deposited and grown on a magnesium oxide film coated on the surface of a silicon single crystal (D. K. Fork, F. A. Ponce, J. C. Tramontana, and T. H. Geballe, Applied Physics Letters, 58, 2294 (1991)). It is also known that magnesium oxide can be used as a diffusion barrier which inhibits the reaction between silicon and barium titanate, and the resulting dielectric material may find use in semiconductor memory devices of the next generation.
Hitherto, there have been reported a number of chemical vapor deposition(CVD) methods for the preparation of a magnesium oxide film at a relatively low temperature.
For example, Kwak et al. reported that a crystalline film of magnesium oxide may be prepared on the surface of a silicon single crystal or quartz by heating bis(2,2,6,6-tetramethyl-3,5-heptanedionato)magnesium of formula 1 to 196.degree. C. and carrying the vapor thereof in an argon stream containing oxygen to the substrate heated above 650.degree. C. (B. S. Kwak, E. P. Boyd, K. Zhang, A. Erbil, and B. Wilkins, Applied Physics Letters, 54, 2542 (1989)). ##STR1##
Lu et al. disclosed that a crystalline film of magnesium oxide can be prepared on the surface of sapphire or strontium titanate at a temperature below 600.degree. C. by carrying bis(2,2,6,6-tetramethyl-3,5-heptanedionato)magnesium in a helium or argon stream to the substrate and then adding oxygen thereto (Z. Lu, R. S. Feigelson, R. K. Route, S. A. DiCarolis, R. Hiskes, and R. D. Jacowitz, Journal of Crystal Growth, 128, 788 (1993)).
Using a plasma-assisted chemical vapor deposition method, Zhao and Suhr prepared crystalline magnesium oxide films on the surfaces of glass, quartz, silicon single crystal and stainless steel by carrying bis(2,2,6,6-tetramethyl-3,5-heptanedionato)magnesium heated at 200.degree. C. in argon stream to the substrate heated above 400.degree. C. and adding oxygen thereto (Y. W. Zhao and H. Suhr, Applied Physics A, 54, 451 (1992)).
Maruyama et al., on the other hand, reported that a crystalline magnesium oxide film may be formed on the surface of glass, quartz or silicon single crystal by treating the substrate heated above 450.degree. C. with an air stream containing magnesium 2-ethylhexanoate of formula 2 (T. Maruyama and J. Shionoya, Japanese Journal of Applied Physics, 29, L810 (1990)). It was reported therein that a magnesium oxide film did not form when the carrier gas was nitrogen instead of air. ##STR2##
According to DeSisto and Henry, an amorphous magnesium oxide film was deposited on the surface of a silicon single crystal, quartz or sapphire by ultrasonic spraying of an ##STR3## aqueous or alcoholic solution of bis(2,4-pentanedionato)magnesium of formula 3 to the substrate heated at 400-550.degree. C. (W. J. DeSisto and R. L. Henry, Applied Physics Letters, 56, 2522 (1990); W. J. DeSisto and R. L. Henry, Journal of Crystal Growth, 109, 314 (1991)). The amorphous magnesium oxide film thus obtained was subsequently converted to a crystalline form by annealing at 700.degree. C. under an oxygen atmosphere.
In the above-mentioned CVD methods, the use of oxygen is essential for the formation of a magnesium oxide film. The magnesium compounds cited above have a magnesium to oxygen atomic ratio of 1:4. As the corresponding ratio in magnesium oxide is 1:1, the formation of a magnesium oxide film must be accompanied by the removal of three equivalent amount of oxygen together with all of the carbon and hydrogen atoms that constitute the organic moieties. It is not well understood at this time exactly how such organomagnesium compounds convert to form a magnesium oxide film in the presence of oxygen, while eliminating the extra oxygen as well as the carbon and hydrogen atoms. However, the magnesium oxide films produced by the prior art methods tend to be contaminated by a significant amount of residual carbon, the residual carbon imparting undesirable effects to the property of the magnesium oxide film.
By using the above-mentioned plasma-assisted CVD method of Zhao and Suhr, a crystalline magnesium oxide film containing little carbon may be obtained below 400.degree. C., the lowest temperature reported in the prior art methods. However, the plasma-assisted CVD method requires a high-power radiofrequency wave generator to produce a plasma as well as sophisticated techniques to generate a uniform plasma over the entire surface of the substrate. Moreover, the plasma CVD method has a serious disadvantage in that the deposition of magnesium oxide occurs only on the surface exposed to the plasma, in contrast to a thermal CVD process wherein film deposition occurs on all surfaces of the substrate. Accordingly, the throughput of plasma-assisted CVD method is much lower than that of thermal CVD method, thus less suitable for use in a large-scale production.
Recently, Auld et al. have reported that a zinc oxide film containing little residual carbon can be coated on the surface of glass heated at 250-400.degree. C. by a chemical vapor deposition method using alkylzinc alkoxides in the absence of oxygen (J. Auld, D. J. Houlton, A. C. Jones, S. A. Rushworth, M. A. Malik, P. O'Brien, and G. W. Critchlow, Journal of Materials Chemistry, 4, 1249 (1994)). This is in line with the results obtained by Ashby et al. that zinc oxide or magnesium oxide is obtained as a by-product when alkylzinc alkoxide or alkylmagnesium alkoxide is pyrolyzed, in accordance with the following reaction paths (E. C. Ashby, G. F. Willard, and A. B. Goel, Journal of Organic Chemistry, 44, 1221 (1979)). EQU R.sup.1 MgOC(CHR.sup.2 R.sup.3)R.sup.4 R.sup.5 - - - .fwdarw. R.sup.1 H+R.sup.2 R.sup.3 C=CR.sup.4 R.sup.5 +MgO EQU R.sup.1 ZnOC(CHR.sup.2 R.sup.3)R.sup.4 R.sup.5 - - - .fwdarw. R.sup.1 H+R.sup.2 R.sup.3 C=CR.sup.4 R.sup.5 +ZnO
A pyrolytic decomposition reaction is proposed to proceed via a unimolecular mechanism involving a six-membered ring transition state, as described below. Ashby et al. reported that sublimation also occurs as the pyrolytic decomposition proceeds when methylmagnesium t-butoxide is heated; ##STR4##
Such alkylmagnesium alkoxide has not been used in the prior art CVD method for the preparation of a magnesium oxide film, presumably because the reaction was unknown to materials scientists of this field until J. Auld mentioned E. C. Ashby et al.'s results in their paper and the alkylmagnesium alkoxides were not readily available. The present inventors nonetheless considered it attractive to use an alkylmagnesium alkoxide as a precursor for a clean, pure magnesium oxide film depositable on a single crystal substrate by a CVD method.